


My Sumu-nii

by Osamuskittycat



Category: Haikyuu!!
Genre: Age Difference, Incest, M/M, miyacest, older Atsumu
Language: English
Status: Completed
Published: 2020-12-08
Updated: 2020-12-08
Packaged: 2021-03-10 04:21:47
Rating: Mature
Warnings: Underage
Chapters: 1
Words: 2,798
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27964496
Author URL: https://archiveofourown.org/users/Osamuskittycat/pseuds/Osamuskittycat
Summary: Atsumu wants to literally shake the hand of the boy that showed his little brother that video and thank him for the best day of his life thus far.
Relationships: Miya Atsumu/Miya Osamu
Comments: 5
Kudos: 255





	My Sumu-nii

**Author's Note:**

> It’s just a mess ok

The first time the words “Sumu-nii” ever left Osamu’s mouth was when he was three years old and absolutely fascinated by his older brother and everything that he did, which never failed to amuse their parents. 

“He’s got Atsumu wrapped around his little finger!”, their mother had said and their father had simply laughed and agreed with his wife because it was completely true that Osamu had his older brother completely wrapped around his little finger.

Atsumu hadn’t really wanted a baby brother, he was his parents' pride and joy so there was no way he was going to share them with some gross little baby that spent all day crying, but then at age ten he was handed a tiny little bundle that was his new baby brother. 

Ten year old Atsumu had stared down at barely a day old Osamu and decided in that exact moment that he was going to be the best big brother in the whole world and that no one would ever hurt his little brother for as long as he lived.

Their parents had been shocked by the change of Atsumu’s attitude towards his new little brother, the older boy taking up the role of big brother with the same drive and enthusiasm he used for volleyball.

There was also a great deal of pride that Atsumu took as Osamu’s big brother, bragging and showing off his little brother to everyone and anyone who would lend him an ear. From his first laugh to his first steps, Atsumu documented every single moment and wouldn’t hesitate to whip his phone out to show off the latest achievement Osamu had managed to do.

Now at twenty-two, Atsumu still took his big brother responsibilities very seriously much to Osamu’s chagrin, that was mostly just for show, he really did love his Sumu-nii despite him being extremely embarrassing sometimes.

One of those moments was right now as Osamu was trying out for the school’s volleyball team and Atsumu had stayed to watch, cheering Osamu on from the bleachers with a tiny pompom that he must've snagged from their mom before coming to tryouts.

Osamu had desperately hoped that no one at practice would recognize his older brother but the gods must have had it out for him because he could definitely hear the whispers, it wasn’t every day that a MSBY Jackals rookie could be seen at a middle school volleyball tryout.

It’s not until after tryouts are done and they’re walking home that Atsumu notices Osamu’s pouting face, his hand holding Atsumu’s hand loosely as they walk, “Okay, grumpy pants. Tell me what’s got your panties in a twist.”

Osamu huffs as he keeps his face turned away from Atsumu but doesn’t let go of his hand, which makes Atsumu bite back a snicker at how adorable his little brother is when he’s being pouty, “Everyone kept staring at you! You’re my Sumu-nii! Not theirs!”

Atsumu blinks in surprise at how annoyed his little brother sounds at the whole ordeal, it’s adorable but Atsumu knows better than to point that out to Osamu, who is now pressed against his brother’s side with both arms around one of his. 

“They kept talking about how they wished that they had a big brother like you and asked me to bring you to practice...But that’s not gonna happen because I don’t wanna share my Sumu-nii with anyone! You’re only mine!”

Before Atsumu can even attempt calming Osamu down, the pre-teen uses his grip on his brother to tug him down just as he gets onto his tiptoes and presses a loud kiss to the corner of Atsumu’s lips, “There! I sealed it with a kiss so no one can take you away now.”

The spot where Osamu’s lips had just kissed, tingles as Atsumu tries to force himself to calm down, head spinning as he tries his hardest not to tip his little brother off on just how much that little kiss ruined his last bit of sanity. 

This wouldn’t be the first time that something Osamu had done scrambled every last brain cell in Atsumu’s head, his little brother seemingly oblivious to the amount of power he had over his big brother. 

The most prominent memory for Atsumu is the time he brought home a girlfriend during his second year at college, Osamu had stood frozen at the door staring silently up at the two young adults holding hands before tears had welled up in his eyes, turning on his heel to run up to his room, door slamming after him loud enough to shake Atsumu’s bones. 

Atsumu’s mother had sighed softly and lead his girlfriend to the living room while he went up to Osamu’s room to try talking his little brother into apologizing for his behavior and to say a proper hello but when Atsumu had knocked on the door of Osamu’s bedroom all he got for his troubles was a loud “I hate you! Go away!” that shattered his heart into a million pieces. 

Osamu had never once said those words to Atsumu, who was sure his little brother probably loved him more than he loved their parents. 

It had taken two days before his brother would speak to him again, but there was still so much tension between the brothers that even their parents had begun to worry. 

It wasn’t until Atsumu broke up with the girl that Osamu returned to his normal self, smiling widely up at Atsumu, having crawled onto his brother’s lap after he’d broken the news about his breakup. 

“Finally, Sumu-nii is all mine again!”

Those words had made Atsumu laugh, had made him ruffle his little brother’s hair before carrying the giggling boy up to his room to play some video games, finally at peace himself now that his baby brother didn’t hate him anymore. 

He’d missed being called ‘Sumu-nii’ by the boy, the coldness he had been forced to endure was gone and was replaced by pure love from his little brother. 

Atsumu should’ve known that all the spoiling he did would come back later to bite him in the ass. 

—

That night after Atsumu had been weaseled by his little brother into giving him a bubble bath, he crawled into his bed and stared up at the ceiling as if the white wall above could give him some answers to the million questions rushing through his head. 

Had he conditioned his little brother to think that he was Osamu’s and no one else’s? Or had his little brother come to that conclusion all by himself? Atsumu didn’t know and really didn’t want to overthink all of this tonight. 

With a tired sigh Atsumu pulled out his phone and scrolled with a random porn site trying to jerk one out in hopes that it would make him tired enough to sleep. 

It’s not until he stumbles upon one of those fake step-siblings porn videos that he notices that it’s two ‘step-brothers’ and when he raises the brightness of his phone he realizes that one of the boys is of a smaller build than the other, dark hair adding to his youthful features along with his beautiful grey eyes that unfortunately remind Atsumu too much of a certain little brother sleeping in the room right next to his. 

He clicks the video before he can change his mind, which was his first mistake. Atsumu’s second mistake is unmuting the video just as the Osamu doppelgänger whines out a soft “nii-san!” which ultimately hammers the last nail into Atsumu’s coffin. 

That night he falls asleep to the memory of Osamu’s sweet voice calling him ‘Sumu-nii’ and soft lips pressed to the corner of his mouth. 

When morning comes he wakes up to a weight against his chest, something tickling his nose as he slowly blinks away the sleep clouding his mind. He’s not surprised when he realizes that the weight on his chest is actually Osamu, who must’ve crawled into his bed after their parents left for work. 

“Baby boy...Come on, get off of me so I can make us breakfast.” 

Osamu simply scrunches up his nose and barrows his face deeper into Atsumu’s chest, lower body pressing down against Atsumu’s groin that is slowly coming to life much to his horror. 

“Come on, ‘Samu. I need you to get off.”

Sadly, Atsumu’s pleads go unheard as Osamu’s legs fall to rest on either side of his brother’s body causing their dicks to press against each other and Atsumu almost throws his baby brother off when he realizes that he’s fully sporting a hard-on just from having his little brother lay over him. 

Atsumu’s hands fall to his brother’s hips, ready to lift him off and get the fuck out of bed and into the bathroom to take of his _problem_ but Osamu finally wakes up, blinking sleepily up at his brother before sitting up to rest his little ass right on top of Atsumu’s ragging hard-on. 

“Sumu-nii? Are you okay?” Hands that have held onto Atsumu’s hands as they cross the street move cup his cheeks as a pout pulls at Osamu’s lips, causing Atsumu to swallow down the urge to kiss it away, “You feel really hot, are you sick?”

‘Sick in the head’ Atsumu thinks to himself as he succumbs to the urge to lean into Osamu’s touch, his own hands moving to rest on his little brother’s hips to squeeze them reassuringly, “I’m fine, Baby boy...I just have a big boy problem that I need to deal with.”

Atsumu instantly cursed himself as curiosity filled Osamu’s eyes as he sat up straight to stare down at his big brother, “Really? What kind of big boy problem, Sumu-nii?” 

Atsumu was going to hell. Straight down to the lowest pits of hell. 

But somehow he didn’t find it in himself to stop. 

“You’re kinda sitting on it, Baby boy.”

Osamu looks confused for a second before Atsumu uses the hands he has on his brother’s hips to gently press him down against the hard cock beneath him, causing the younger boy to gasp softly at the girth pressed up against his own little cock. 

“O-Oh...Sumu-nii’s thingy is hard.”

Atsumu would’ve laughed at his brother’s sweet innocence if it hadn’t turned him on even more, the absolute wonder in his little brother’s eyes enough to push Atsumu forward, “It’s called a ‘cock’, Baby. You can call it a cock when you’re with me.”

Osamu bites his lip as he rubs down against Atsumu to get another feel of the hard cock beneath him, blush high on his cheeks as he grips onto his big brother’s shirt tightly, “S-Sumu-nii’s c-cock is...is really b-big…”

Atsumu doesn’t bite back his groan as he slips a hand under Osamu’s shirt to brush his finger tips against skin he knows almost as well as his own, “It is, Baby...Do you wanna see it?”

Osamu’s silent as he nods his head, eyes widening when Atsumu manhandles him as he sits up to lean back against the headboard before settling Osamu between his spread knees. 

“Okay, Baby. Why don’t you pull down Sumu-nii’s pants and take a look at how big he is.”

It seems Atsumu doesn’t have to repeat himself twice because the second the permission is out of his mouth, Osamu’s on him in an instant, his hands grabbing at his brother’s sweats to pull them down, gasping softly at the way Atsumu’s hard cock slips out. 

“W-Whoa...it’s so _big_!”

Atsumu reaches out to take Osamu’s hand and helps wrap it around his cock, hissing softly at the contact before he settles back to watch Osamu explore and touch to his heart’s content. 

“C-Can I...Can I put it in my mouth? I really wanna give you a blowjob.”

A record scratch sound goes off in Atsumu’s head, his head snapping up to look at his not so innocent little brother, who is blushing so beautifully while biting his lower lip, staring at Atsumu from under his lashes. 

“Where the hell did you learn about that?”

If possible, Osamu’s face gets darker as he looks down at the hand around Atsumu’s cock, thumb brushing away the pre-cum pooling on the head. 

“One of the guys on the team showed us a video of a girl giving a guy a blowjob...he said that a lot of guys love them and that it makes them feel good. I wanna make Sumu-nii feel good!”

Atsumu can’t believe this is happening, he has to be dreaming because there’s not a damn chance his sweet baby brother just offered to give him a blowjob just now, “A-Are you sure, Baby? I’m pretty big and I don’t wanna hurt you.”

Osamu gets that adorable pout back as he squeezes Atsumu’s cock lightly without realizing it, “I’ve been practicing! I can take a whole banana now without feeling like I’m gonna puke! I swear!”

Forget hell, Atsumu is sure he’s in heaven right about now. He should be upset that his little brother is learning about sex from pornos but the boy looks extremely determined to prove to his older brother than he knows what he’s doing and who is Atsumu to deny his baby brother something he so desperately wants?

“You’ll go slow and if you feel like it’s too much at any point, I want you to stop. I promise I won’t be upset if you can’t keep going. 

Osamu’s smile hits Atsumu right in the chest, his little brother swooping in to kiss Atsumu right on the lips before he wiggles his way down until he’s laid out comfortably between his big brother’s legs. 

“Thank you, Sumu-nii!”

Atsumu doesn’t think he’ll ever be able to hear Osamu call him ‘Sumu-nii’ without getting hard but he’s not too upset about it.

“Okay, Baby. Show big brother what that pretty little mouth can do.”

Osamu wasn’t kidding when he said that he’s practiced, he still has a long way to go (and Atsumu can’t wait to show him the ropes) but he’s got enthusiasm and Atsumu’s desperate need to cum on his side. 

Atsumu keeps a hand in his little brother’s hair to keep Osamu from taking more than what he’s able to despite him showing no discomfort yet which settles some of Atsumu’s nerves. 

There’s no technique but Osamu’s drive is enough to get Atsumu close to cumming, the wet noise of his brother bobbing up and down his cock an added bonus to an already great blowjob. 

“Baby...Sumu-nii is gonna cum soon, why don’t you pull off?”

Osamu pulls off of Atsumu’s cock with a soft pop that’ll forever be burned into Atsumu’s brain and shakes his head as he tries to catch his breath, tiny hips rocking down against the bed, “But I wanna taste Sumu-nii’s cum!”

Atsumu wants to literally shake the hand of the boy that showed his little brother that video and thank him for the best day of his life thus far. 

After taking a solid breath in, Atsumu reaches out to gently cup Osamu’s cheek, thumb pressing down against his little brother’s bottom lip with a soft grin, “Okay, Baby. It tastes a little weird so it’s okay to spit it out okay?”

Osamu presses a tiny kiss to Atsumu’s thumb, surprising his brother before diving back down to swallow his brother’s cock like he’s hungry for it, desperately trying to make Atsumu cum now. 

It takes watching Osamu slip a hand between himself and the bed for Atsumu to choke out a warning before he’s flooding his baby brother’s mouth with cum, eyes wide as he feels the younger boy suckle on the head as if he’s trying to milk Atsumu’s cock for every last drop. 

When Atsumu can’t take anymore, he slowly pulls his brother off his cock to sit him on his lap, biting back a groan as he watches Osamu lick his lips happily as if he’s just had a piece of candy and not his brother’s cum in his mouth. 

“That was so much fun, Sumu-nii! And your cum tastes so good!”

Atsumu’s cock gives a weak twitch as he slips his hand into Osamu’s sleep shorts to wrap a hand around his little brother’s own hard cock, giving it a gentle squeeze just to see the way his sweet boy’s eyes flutter, “Thank you for that, Baby boy. That felt really good...you’ll be a pro at this soon enough. But only for Sumu-nii, right baby boy?”

Those words make Osamu’s hips twitch, rocking up into Atsumu’s hand as he whines softly as he feels Atsumu’s other hand sneak around to press a spit wet finger against his little hole. 

“O-Only for Sumu-nii...My Sumu-nii.”

  
  


**Author's Note:**

> Follow my Twitter: @osamuskittycat


End file.
